The host response to blood borne pathogenic organisms involves the responses facilitated by the marginal zone where pathogens first come in contact with cells of the immune system. The antibody response elicited by the marginal zone upon exposure to encapsulated microorganisms is dominated by the mouse IgG3 subclass in a T independent and complement C3 dependent reaction. Our work has focused on the genetic pathways that contribute to the organization of the marginal zone and the cellular responses that are triggered by this anatomic structure upon exposure to encapsulated microbial pathogens. Two main areas of investigation will be pursued in this proposal for the next granting period - 1) characterization of a novel FcgammaR that engages IgG3, called FcgammaRIV, and its role in the host response to encapsulated microbial pathogens and 2) dissection of the mechanisms that govern the coordinated movement of marginal zone B cells and macrophages in response to microbial challenge through the analysis of specific genetic pathways, such as pyk2 and SHIP, that we have identified as being critical in the organization and function of the marginal zone. To accomplish these broad goals, four specific aims will be addressed: 1) Define the structure, function and expression of FcgammaRIV on myeloid and lymphoid cells. Experiments are proposed to define the expression and regulation on various cell types, the structural basis for its IgG3 binding specificity, the subunit composition and signaling properties of the receptor and its interactions with complement pathways and inhibitory signaling responses. 2) Analyze the in vivo role of FcgammaRIVin host response to encapsulated pathogens. Mice with conditional deficiency of FcgammaRIV in either lymphoid or myeloid cells will be constructed, combined with similar deficiencies in FcgammaRI, II or III and studied in both thymic dependent and thymic independent responses. Passive or active pathogen specific anti-capsular polysaccharide responses will be generated in these strains and the resulting animals challenged with S. pneumoniae and C. neoformans to determine the role of specific FcRs in protection or enhancement. 3) Characterize the molecular interactions between marginal zone macrophages and marginal zone B cells. The endogenous ligand for the MARCO scavenger receptor on marginal zone B cells will be identified and characterized for its expression and function in vitro and in vivo. The role of this receptor/ligand pair in marginal zone organization and response to capsular polysaccharides will be pursued. 4) Determine the mechanism of cellular migration in the marginal zone in response to microbial challenge. Pyk2 and SHIP conditional mutants will be analyzed for their role in marginal zone B cell migration, retention and survival. The role of specific macrophages subsets will be investigated in the B cell and macrophage movement triggered by S. pneumoniae and S. aureus. These studies will clarify the host response to encapsulated microbial pathogens through the analysis of the marginal zone and specific antibody response elicited by these pathogens.